Characterizing the Spatial and Temporal Inundation of Webbs Marsh, Murderkill River Estuary

The purpose of the project is to characterize the spatial and temporal inundation of a salt marsh in the Murderkill River Estuary and to determine the feasibility of using heat as a tracer of flow to characterize inundation of other marshes in the estuary. The project will contribute to a more robust parameterization of river-marsh interaction in the water-quality model that is being developed for the Murderkill River by Kent County.

Tidal inundation of a salt-marsh platform is an extremely dynamic process that remains poorly understood even though it is of primary importance in understanding nutrient cycling and ecological zonation in estuaries. This is partly due to the sampling requirements to fully describe a shallow flow system with high temporal and spatial variability. This project is designed to make the redundant observations necessary to unambiguously capture the spatial and temporal variability in tidal inundation of Webbs Marsh, determine the locations of groundwater and surface-water influxes and effluxes to Webbs Marsh, and delineate a watershed boundary for Webbs Slough.

The results will be used to evaluate the feasibility of using a smaller suite of measurements to characterize inundation of other marshes in the estuary. The focus will be on using heat as the primary tracer of flow and as an inundation indicator because heat transfer is tightly coupled with hydrologic processes, temperature loggers are inexpensive and easy to deploy, and a thermal-infrared radiometer allows for remote temperature measurements over broad areas. The latter facilitates quick field testing of conceptual models and choosing representative areas for placing instrumentation. The results will complement other studies proposed for Webbs Marsh on nutrient cycling, sediment-oxygen demand, and ecological zonation that are strongly influenced by tidal inundation.

The characterization of Webbs Marsh will include:

1. elevation, relief, and area of marsh platform and levees,
2. frequency, duration, and depth of inundation of the marsh platform during neap, spring, and normal tides,
3. delineation of the topographic watershed boundary for Webbs Slough and evaluation of potential hydrodynamic boundaries,
4. location and character of surface water exchange with Brockonbridge Gut, and
5. locations of surface water inflows from uplands

The characterization and feasibility of transferring the methodology to other marshes in the Murderkill Estuary will be based on synthesis of:

1. remotely sensed water and sediment temperatures (ground-based and aerial for spatial coverage and over a tidal cycle at select locations for temporal coverage),
2. time series of water temperature, salinity, and pressure from an array of data loggers (temporal coverage),
3. time series of solar radiation, air temperature and humidity, and wind speed and direction from an automated weather station,
4. time series of water velocity, discharge, elevation, temperature, and salinity from the existing USGS gauging station at the mouth of Webbs Slough.
5. high-resolution elevation maps (aerial LIDAR and ground-based GPS surveys)

The project, which is being conducted in cooperation with Kent County, will support work by Kent County to evaluate the nutrient TMDLs for the tidal portion of the Murderkill River.